Implantable medical electrodes are used in a variety of medical applications. One such application is the sensing of cortical electrical activity which can be analyzed to identify the foci of epileptogenic brain for removal. The same implantable medical electrodes which are used to sense cortical electrical activity passively can also be used to stimulate various regions of the brain to further analyze the foci of epileptogenic brain in order to enhance the safety and effectiveness of epileptogenic brain removal.
One conventional type of implantable medical electrode used for sensing cortical electrical activity is a depth electrode which is a relatively narrow, typically cylindrical structure with conductive ring electrodes spaced along its length. A depth electrode is an intracortical device that is inserted into the brain tissue. Depth electrodes provide electrical contact to, and thus information regarding electrical activity within the brain itself.
Another type of implantable medical electrode for use in sensing cortical electrical activity is referred to as a strip electrode. A strip electrode is inserted between the dura and the cortex and does not penetrate the brain. Strip electrodes typically include a flexible, substantially flat strip of dielectric material supporting one or more flat electrical contacts with which cortical electrical activity on the surface of the brain is stimulated and/or sensed. Each flat contact is connected to a proximal end of an insulated lead wire having a distal end suitable for coupling to electrical stimulation and/or monitoring apparatus. It is important that the strip electrode be flexible in order to conform to the patient's cortex.
More particularly, strip electrodes generally include two dielectric layers between which the flat electrical contacts are located. One of the dielectric layers has a plurality of apertures therethrough, with each aperture aligned with a corresponding contact so as to expose at least a portion of the contact.
It is critical that each of the flat contacts comes into contact with the cortex and, once in contact, remains in the same fixed position relative to the cortex. Knowledge of the exact positions of the strip electrode contacts relative to the cortex is necessary in order to properly interpret the electrical readings.
Conventionally, providing a strip electrode with a certain amount of thickness, such as on the order of 0.020-0.030 inches, has been felt to maintain adequate positioning of the electrode once implanted. This thickness has also been felt to enhance support of the lead wires by preventing them from breaking away from the contacts and/or becoming dislodged within the strip electrode. However, it is also desirable to make the strip electrode relatively thin in order to avoid raising intracranial pressure when the dura is closed, particularly in pediatric cases, in which there is only a relatively narrow space between the dura and the brain.
Another type of conventional medical electrode is similar to the strip electrode in construction, but includes an array of electrical contacts. Thus, such an electrode generally includes two dielectric layers between which a plurality of flat electrical contacts are arranged in the form of a two-dimensional array with at least a portion of each contact exposed by an aperture in one of the dielectric layers.